The removal of carbonyl sulfide (COS) from mixtures of gases by liquid absorbents is an important industrial operation. Refinery and synthetic gases, derived from either petroleum fractions or coal, often contain significant amounts of COS. The manufacture of olefins, notably C.sub.2 H.sub.4 and C.sub.3 H.sub.6, from petroleum fractions also entails absorption of COS because of the close boiling points of COS and C.sub.3 H.sub.6. It is necessary to remove COS down to a few ppm for several reasons, such as catalysts sensitivity to COS in subsequent operations, statutory regulations regarding sulfur content in vent gas, and corrosion aspects of sulfur compounds in pipelines. In addition, the presence of COS has been identified as the cause of nonreversible degradation reactions in several commercial acid gas removal processes. This necessitates additional capital and energy costs for reclaiming and/or replacing the resulting spent solvent.
COS can be absorbed along with H.sub.2 S and CO.sub.2 in a variety of chemical and physical solvents. Sykes, U.S. Pat. No. 3,965,244, Bozzelli, et al. U.S. Pat. No. 4,100,256, and U.S. Pat. No. 4,112,049, all teach the use of chemical solvents to hydrolize COS. The prime examples of chemical solvents are aqueous solutions of primary and secondary amines such as monoethanol amine (MEA) and diethanol amine (DEA), respectively. While COS can be removed from the gas effectively by these chemical solvents, it generally degrades the solvents by forming undesirable stable compounds such as thiocarbonates, as in the case of MEA and DEA. Substantial thermal energy is required to regenerate the spent solvents, thereby increasing processing costs.
The trend in the art has been to employ physical solvents in place of these chemical solvents. Physical solvents do not have the disadvantage of forming undesirable stable compounds as discussed above, and can absorb more gas under pressure than chemical solvents. Physical solvents such as polyethylene glycol dimethyl ether, sold under the tradename Selexol, and cold methanol, sold under the tradename Rectisol, remove acid gases based on the principle of physical absorption, i.e. Henry's Law. When used alone however, physical solvents are often inadequate, especially when used in coal gasification operations where large amounts of COS are present.
To overcome this drawback, current commercial practice is to effect gas phase COS hydrolysis over a suitable catalyst. Catalysts such as Pt on Al.sub.2 O.sub.3 have been employed for this hydrolysis. For example, COS will hydrolyze partially over CO shift catalysts in the presence of sufficient steam (0.1 mole steam/mole of gas) at high temperatures; e.g., 120.degree.-300.degree. C. There are two problems with this type of process however. First, COS hydrolysis is incomplete and limited by the equilibrium of the reaction if H.sub.2 S and CO.sub.2 are not removed. Second, if H.sub.2 S and CO.sub.2 are removed first at lower temperature, the gas stream would have to be heated up for COS hydrolysis, followed by another step for H.sub.2 S removal. This procedure is costly due to the large energy requirement.
European patent application No. 0,008,449 discloses adding a monocyclic amine catalyst to an aqueous solvent to effect COS hydrolysis. Operation of this method however requires a high concentration of catalyst; up to 90% for example; and is only effective when small amounts of COS are present. This method also has the disadvantage in that unwanted salts tend to form from the contact of the hydrolysis products with the excess monocyclic catalysts.
U.S. Pat. Nos. 3,966,875 and 4,011,066 disclose using homogeneous catalysts in physical acid gas removal solvents. These references however only disclose using mono-cyclic amine catalysts such as 1,2-dimethylimidazole, and teach using separate hydrolysis and absorption towers. These mono-cyclic catalysts have only moderate activity for COS hydrolysis.
Holoman, et al. U.S. Pat. No. 4,096,085 discloses adding a bicyclo tertiary amine to an acid gas scrubbing system. This reference teaches adding a small amount of bicyclo amine to a chemical solvent to inhibit corrosion in the system. We have demonstrated that it takes a larger concentration of the bicyclic amine than is disclosed in this reference to effect COS hydrolysis. In addition, Holoman only teaches adding these compounds to chemical acid gas removal solvents.